Optical memory systems, which use an optical disc such as a compact disc (CD), a DVD or a BD (Blu-ray Disc) or an optical card as an information recording medium, have been used extensively as optical information reproduction/recording apparatuses.
To realize a storage capacity that is big enough to store a huge amount of information, a lot of people think it a good idea to use a multilayer information recording medium that has multiple recording layers. However, as the number of recording layers stacked in a single recording medium increases, the light needs to pass through the many recording layers to get a recording operation done on the deeper target recording layer. Thereby, the intensity of the light drops to a lower value when it reaches the target. As a result, the recording operation could not be performed as intended or the number of recording layers stacked could not be increased anymore. In this case, the “shallowest” recording layer is supposed to be located closest to the objective lens and the “deepest” recording layer is supposed to be located most distant from the objective lens.
To overcome such a problem, a multilayer optical recording medium, of which each recording layer is made up of a variable absorption film and a transparent recording film arranged adjacent to the variable absorption film, and an apparatus for reproduction and/or recording from/to it have been proposed (see Patent Document No. 1). In the variable absorption film, an absorption edge shifting phenomenon that the absorption edge of the absorption spectrum will shift to a longer wavelength range as the temperature rises is observed.
Among the multiple recording layers, the incident light for recording is condensed on the target recording layer. That is why the temperature of the variable absorption film rises, and its absorptance with respect to the incident light for recording increases, thus generating heat, which is then transferred to the transparent recording film. As a result, a recording operation can get done on it with good sensitivity. With respect to every recording layer but the target layer, however, the light for recording is defocused, and therefore, the temperature of its variable absorption film hardly rises, and the decrease in the transmittance of the recording layer can be minimized. As a result, according to Patent Document No. 1, recording energy that is high enough to get a recording operation done successfully is secured for even the deepest recording layer, and a recording operation could get done as intended even on a multilayer information recording medium with five to ten layers.                Patent Document No. 1: Pamphlet of PCT International Application Publication No. 03/102941        